Then it seems that should be on your website, instead of listing:
"Unsurpassed file compatibility
The best PSD import engine available and full support for SVG, EPS, PDF, PDF/X and FH files means injecting Affinity Designer into your workflow is painless."
I can appreciate the notion of having non-supported features rasterized, but that's not exactly full support or painless, if you need to work with to what you're importing beyond it being just an image. The expectation that provides isn't accurate.

My guess is that it's likely related to something like this: http://www.pcworld.com/article/2875740/nvidia-explains-geforce-gtx-970s-memory-performance-issues-admits-error-in-specs.html
Considering those problems, and the fact that copying puts the image or whatever into memory, then it may be going straight to VRAM rather than straight RAM.
This may also be useful:
http://www.tomshardware.com/forum/id-2765450/geforce-gtx-970-driver-problems.html
https://www.gamespot.com/articles/nvidia-admits-to-error-in-gtx-970-specs-and-memory/1100-6424915/
https://www.extremetech.com/extreme/199684-nvidia-slapped-with-class-action-lawsuit-over-gtx-970-memory-issues
http://www.tomshardware.com/answers/id-3141196/gtx-970-memory-bug-fixed.html
https://www.pcper.com/news/Graphics-Cards/NVIDIA-Responds-GTX-970-35GB-Memory-Issue
Have you updated your graphics driver recently? If not, you may want to. If so, you might want to roll back to an earlier driver. Having said that, it may just be a hardware issue that can't be solved...assuming it's your card.
I also noticed that you've overclocked your CPU. Try setting it to default speeds and see what happens.

This program is amazingly unintuitive. It's one of the main reasons I feel like I can't just jump ship from Photoshop or Illustrator to Photo or Designer.
For example, simple things like filling a selection should be reasonably intuitive. I realize it's hard to target everyone's train of thought, but here's mine:
1) Open an image in Designer
2) Make a selection of what you want to fill
3) Use the 'Fill Tool' and draw in a fill within that selection based on whatever 'Fill Tool' options I've chosen.
However, all that happens is that I get a line that looks like Photo's 'Gradient Tool,' but nothing happens. I go to Tutorials under the Help Menu, which takes me to a website, where I do a search for Fill Tool and I'm greeted with a myriad of things that don't apply. So, I go to Help... under the Help Menu and look up Fill Tool and I'm greeted with the list of information below (which is neither descriptive enough to be useful, nor simple enough in application to be effective).
Considering the first two part of this list of information, where the fill tool allows you to adjust the fill, then I expect to be able to adjust from no fill to a fill and to apply the flood with a single click, but that never happens.
Taking this a step further, if I have a selection made and try to paint with the vector brush inside that selection, the paint doesn't obey the boundaries of the selection. What's the point of the selection if it doesn't set boundaries?
Search results for fill tool
Fill Tool
The Fill Tool allows you to adjust the fill and line colors applied to vector and text objects.
Flood Fill
The Flood Fill tool allows you to fill in areas of your page, selection, or object with a single click.
Selecting colors
There are various ways to select and apply color in Affinity apps.
Swatches panel
The Swatches panel stores a selection of color, gradient and recently used document color swatches for applying to objects.
Gradient editing
Customize gradients for object fills and lines.
Keyboard shortcuts
A list of commonly used shortcuts.
Color panel
The Color panel is used to apply color to objects.
About Lines and Shapes
Open curves and closed shapes can be created and are controlled by nodes and control handles.
Artboard color and opacity
Discussion of artboard color properties.
Transparency Tool
The Transparency Tool allows you to apply and edit transparency gradients to vector and text objects.
Outer Shadow
The Outer Shadow effect is used to add a shadow to edges.
Transparency
Find out about transparency and how it is applied to your design.
Pen Tool
The Pen tool is used to draw very precise curves and shapes.
Pencil Tool
The Pencil Tool allows you to create hand-drawn lines and shapes.
Sampling colors
Pick up colors from objects on your page or pixels on your screen.
Transparency Editor
The Transparency Editor dialog lets you customize transparency gradients for object fills and strokes.
Move Tool
The Move Tool allows you to select, move, rotate, and resize objects.
Layer masking
Layer masks allow you to selectively hide layer contents to reveal layers below the masked layer.
Ellipse Tool
The Ellipse Tool takes the hassle out of creating circles and ellipses.
Corner Tool
The Corner tool rounds sharp corners on closed shapes.
Inner Shadow
The Inner Shadow effect is used to add a diffused shadow to the inside of an object.
Key features
An overview of Affinity Designer key features.
Color Picker Tool
The Color Picker Tool samples colors from anywhere on your document or screen.
Frame text
Use frame text for paragraphs with a formalized structure and layout.

This isn't likely to ever happen for a couple of reasons. Making an app portable like that simply means it's very easy to copy to whomever without them paying for it. Also, when you connect a drive to a computer, you may find that it assigns that drive a letter or volume inconsistent from the path the file was saved to or open from before, potentially causing problems.

This should support import/export of DrawPlus files, especially since DrawPlus was made by the same company. I can't currently open a .dpp file that I can in DrawPlus in any of the Affinity projects to carry them over.

I had to come here to try to figure this out...read a few replies and still going to determine how to simply fill a selection with color. This program is amazingly unintuitive. It's one of the main reasons I feel like I can't just jump ship from Photoshop or Illustrator to Photo or Designer. Simple things like filling a selection should be reasonably intuitive. I realize it's hard to target everyone's train of thought, but here's mine:
1) Open an image in Designer
2) Make a selection of what you want to fill
3) Use the 'Fill Tool' and draw in a fill within that selection based on whatever 'Fill Tool' options I've chosen.
However, all that happens is that I get a line that looks like Photo's 'Gradient Tool,' but nothing happens. I go to Tutorials under the Help Menu, which takes me to a website, where I do a search for Fill Tool and I'm greeted with a myriad of things that don't apply. So, I go to Help... under the Help Menu and look up Fill Tool and I'm greeted with the list of information below (which is neither descriptive enough to be useful, nor simple enough in application to be effective).
Considering the first two part of this list of information, where the fill tool allows you to adjust the fill, then I expect to be able to adjust from no fill to a fill and to apply the flood with a single click, but that never happens.
Taking this a step further, if I have a selection made and try to paint with the vector brush inside that selection, the paint doesn't obey the boundaries of the selection. What's the point of the selection if it doesn't set boundaries?
Search results for fill tool
Fill Tool
The Fill Tool allows you to adjust the fill and line colors applied to vector and text objects.
Flood Fill
The Flood Fill tool allows you to fill in areas of your page, selection, or object with a single click.
Selecting colors
There are various ways to select and apply color in Affinity apps.
Swatches panel
The Swatches panel stores a selection of color, gradient and recently used document color swatches for applying to objects.
Gradient editing
Customize gradients for object fills and lines.
Keyboard shortcuts
A list of commonly used shortcuts.
Color panel
The Color panel is used to apply color to objects.
About Lines and Shapes
Open curves and closed shapes can be created and are controlled by nodes and control handles.
Artboard color and opacity
Discussion of artboard color properties.
Transparency Tool
The Transparency Tool allows you to apply and edit transparency gradients to vector and text objects.
Outer Shadow
The Outer Shadow effect is used to add a shadow to edges.
Transparency
Find out about transparency and how it is applied to your design.
Pen Tool
The Pen tool is used to draw very precise curves and shapes.
Pencil Tool
The Pencil Tool allows you to create hand-drawn lines and shapes.
Sampling colors
Pick up colors from objects on your page or pixels on your screen.
Transparency Editor
The Transparency Editor dialog lets you customize transparency gradients for object fills and strokes.
Move Tool
The Move Tool allows you to select, move, rotate, and resize objects.
Layer masking
Layer masks allow you to selectively hide layer contents to reveal layers below the masked layer.
Ellipse Tool
The Ellipse Tool takes the hassle out of creating circles and ellipses.
Corner Tool
The Corner tool rounds sharp corners on closed shapes.
Inner Shadow
The Inner Shadow effect is used to add a diffused shadow to the inside of an object.
Key features
An overview of Affinity Designer key features.
Color Picker Tool
The Color Picker Tool samples colors from anywhere on your document or screen.
Frame text
Use frame text for paragraphs with a formalized structure and layout.

I'm never clear on which of these (Designer vs Photo) are directly aimed at competing with Photoshop, but even Photoshop can do simple animated gifs, etc. I would think that's a feature that should be added.

Is there a way to adjust a selection? For instance, I may try to draw a circle around an image of a circle, but not get it quite right, only to find that I can't adjust it after the selection is drawn...causing me to have to keep trying to draw it perfectly and wasting a ton of time.

This really needs to be improved. Photoshop and some other programs have the ability to detect the edge of objects in an image, such that if you had a very defined contrast between the object and a surrounding background, you can easily select just the object and/or flip between it and the background. This is extremely useful for removing backgrounds or objects.
Ah...wait, I just discovered the flood select tool. That's what I'm looking for. It's not perfect and certainly could be improved in its accuracy, but definitely helpful.